The present invention relates generally to the preparation of semiconductor grade single crystal silicon which is used in the manufacture of electronic components. More particularly, the present invention relates to vacancy dominated single crystal silicon ingots and wafers that are doped with nitrogen to stabilize oxygen precipitation nuclei and are substantially free of oxidation induced stacking faults.
Single crystal silicon, from which a single crystal silicon wafer may be obtained, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter is typically reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
It is now recognized that a number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects, which are known as crystal lattice vacancies (“V”) and silicon self-interstitials (“I”). The type and initial concentration of the intrinsic point defects are determined at the time of solidification and, if these concentrations reach a level of critical supersaturation in the system and the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur. Agglomerated intrinsic point defects in silicon can severely impact the yield potential of the material in the production of complex and highly integrated circuits.
Agglomerated vacancy-type defects include such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Crystal Originated Particle (COP) Defects, crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques such as Scanning Infrared Microscopy and Laser Scanning Tomography. COPs are of particular interest because Gate Oxide Integrity failures correlate to the concentration of COPs on the wafer surface. D. Graf, M. Suhren, U. Schmilke, A. Ehlert, W. v. Ammon and P. Wagner., J. Electrochem. Soc. 1998, 145, 275; M. Tamatsuka, T. Sasaki, K. Hagimoto and G. A. Rozgonyi, Proc. 6th. Int. Symp. On Ultralarge Scale Integration Science and Technology “ULSI Science and Technology/1997,” The Electrochemical Society 1997, PV 97-3, p. 183; and T. Abe, Electrochem. Soc. Proc. 1998, PV 98-1, 157; N. Adachi, T. Hisatomi, M. Sano, H. Tsuya, J. Electrochem. Soc. 2000, 147, 350.COPs within an ingot or wafer are octahedral voids. At the surface of a wafer, the COPs appear as pits with silicon dioxide covered walls and are typically about 50–300 nm wide and can be up to about 300 nm deep.
Also present in regions where vacancies are present but where agglomeration has not occurred, are defects which act as the nuclei for oxidation induced stacking faults (OISF). It is speculated that this particular defect, generally formed proximate the V/I boundary, is a high temperature nucleated oxygen precipitate catalyzed by the presence of the non-agglomerated vacancies (“free vacancies”); that is, it is speculated that this defect results from an interaction between oxygen and free vacancies in a region near the V/I boundary.
Defects relating to self-interstitials are less well studied. Agglomerated interstitial-type defects include B-defects which are generally regarded as interstitial clusters and I-defects which are generally regarded as dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
One approach to dealing with the problem of agglomerated intrinsic point defects includes growing the silicon crystal ingot at a high rate in an attempt to cause the ingot to be “vacancy dominated” (i.e., silicon wherein vacancies are the predominant intrinsic point defect) and then epitaxially depositing a thin crystalline layer of silicon on the surface of the vacancy dominated single crystal silicon wafer effectively filling or covering the agglomerated vacancy defects. The epitaxial deposition process typically involves a chemical vapor deposition process wherein a single crystal silicon wafer is rapidly heated to a temperature of about 1150° C. while a gaseous silicon compound is passed over the wafer surface to effect pyrolysis or decomposition. Although, this process provides a single crystal silicon wafer having a surface which is substantially free of agglomerated vacancy defects, it also annihilates oxygen precipitation nuclei formed during the growth of the ingot. Oxygen precipitation nuclei are necessary for the formation of oxygen precipitates during subsequent thermal processing associated with electronic device fabrication. The oxygen precipitates act as gettering sites for capturing metallic impurities in the bulk of the wafer and away from the surface. Without the ability to getter metallic impurities, the electronic properties of the wafer may be negatively impacted; for example, the wafer may have a decreased minority carrier lifetime, current leakage at p-n junctions, dielectric constant discontinuity and reduced breakdown strength.
One method for dealing with the problem of annihilating oxygen precipitation nuclei during epitaxial deposition is a lengthy thermal annealing process (e.g., about 4 hours at about 800° C. followed by 10 hours at about 1000° C.) to stabilize the oxygen precipitation nuclei against the rapid thermal epitaxial deposition process. This method decreases throughput and significantly increases the cost of manufacturing the silicon wafers.
A second method is to stabilize the oxygen precipitation nuclei with nitrogen doping of the silicon crystal (see, e.g., F. Shimura et al., Appl. Phys. Left. 48 (3), p. 224, 1986). Specifically, F. Shimura et al. disclosed that the oxygen precipitation nuclei in a nitrogen doped crystal are stable up to about 1250° C. For example, nitrogen doping was recently reported to produce an epitaxial silicon wafer substrate with high gettering capability (see Japanese Patent Office Publication Number 1999-189493). However, the high gettering capability was due in part to the nearly uniform distribution of OISF throughout the wafer which negatively impacts the quality of the epitaxial wafer. Specifically, OISF on the surface of a silicon wafer, unlike other vacancy-type defects, are not covered by the deposition of an epitaxial silicon layer. OISF continue to grow through the epitaxial layer and result in grown-in defects commonly referred to as epitaxial stacking faults. Epitaxial stacking faults have a maximum cross-sectional width ranging from the current detection limit of a laser-based auto-inspection device of about 0.1 μm to greater than about 10 μm.
Accordingly, a need continues to exist for a process to grow a single crystal silicon ingot that comprises a portion or segment that is predominantly vacancy dominated and contains stabilized oxygen precipitation nuclei and is substantially free of OISF. Such an ingot segment would yield substrate wafers particularly suited for epitaxial deposition.